This invention relates to the use of certain thiomorpholine compounds in sulfur-vulcanized rubber reinforced with a siliceous pigment and a coupling agent. It more particularly pertains to the use of thiomorpholines such as 4,4'-dithiodimorpholine and 2-(4-morpholinothio)-benzothiazole for retarding scorch and improving vulcanizate properties of a rubber compound containing siliceous pigment and a polyfunctional coupling agent having a functional group capable of bonding to the siliceous pigment and a sulfur functional group, such as a mercapto group, capable of bonding to the rubber polymer. It pertains also to rubber compounds containing silica, a coupling agent, and a thiomorpholine scorch retardant, and to vulcanizates of such compounds.
When reinforcing siliceous pigments such as pyrogenic and precipitated silicas are used as fillers in rubber compounds, they give highly desirable characteristics to the resulting vulcanizate. Rubber vulcanizates filled with silica have good resistance to tearing, cutting, flex cracking, and aging, and in addition can be made in white or light colors.
Certain coupling agents are known which strengthen adhesion between the siliceous pigment and the rubber polymer. The coupling agents are generally bifunctional compounds capable of bonding to the siliceous particle and to the rubber polymer under rubber compounding and vulcanizing conditions. The bond to the silica particle is preferably through a silicon-containing group capable of reacting with the silanol groups covering the surface of the particle. The bond to the rubber polymer is preferably through a mercapto group on the coupling agent.
Preferred coupling agents are the mercaptoalkyl silanes, particularly mercaptoalkyltrialkoxysilanes such as 3-mercaptopropyltrimethoxysilane and 2-mercaptoethyltriethoxysilane. Rubber vulcanizates reinforced with silica and these coupling agents show better stress/strain properties, lower heat buildup and permanent set, and greater resistance to abrasion than rubbers reinforced only with silica.
Sulfur-vulcanized rubber compounds containing silica tend to be rather slow curing. Mercapto-type coupling agents, especially in higher quantities, increase the rate of cure but unfortunately can reduce scorch time to unacceptable levels. They are also relatively costly. The tendency of a rubber compound to scorch makes compounding and processing more difficult. Mixing and milling must be done more quickly yet at lower temperatures so that the compound will not begin to vulcanize before it is shaped or molded. Scorch can render a rubber compound stiff, lumpy, and useless. A rubber compound containing silica and a mercaptotype coupling agent, yet having a relatively long scorch time and a reasonably fast rate of cure, would be especially desirable.
A recognized method for increasing the rate of cure is to add a secondary accelerator, commonly a guanidine derivative such as diphenylguanidine, in addition to sulfur and an accelerator. Secondary accelerators do increase the cure rate, but they also can reduce scorch time. Using conventional accelerator adjustments in formulating a fast-curing rubber compound containing silica and a mercapto-type coupling agent, unacceptably short scorch times were encountered.
Thiomorpholine accelerators are well known in the art for use in sulfur-vulcanized rubber compounds. Two recent patents, U.S. Pat. Nos. 3,852,250 and 3,852,251, disclose the use of thiomorpholine accelerators in combination with benzothiazole disulfide or sulfenamide accelerators and sulfur to provide vulcanizates with improved physical properties. The cited patents neither disclose the use of coupling agents nor the problem of excessive scorchiness associated with the use mercaptotype coupling agents in silica-filled, sulfur-vulcanizable rubber.